In the VFFS process, a flat web of synthetic thermoplastic film is unwound from a roll and formed into a continuous tube in a tube-forming section. In the next step, the longitudinal edges of the film are sealed together to form a “lap seal” or a “fin seal.” The tube then is pulled vertically downwards to a filling station and collapsed across its transverse cross-section. The position of such cross-section is at a sealing device below the filling station, which makes an air-tight transverse heat seal at the collapsed portion of the tube.
Next, the flowable material to be packaged enters the tube above the transverse heat seal, continuously or intermittently, filling the tube upwardly from the transverse heat seal. The tube is then allowed to drop a predetermined distance usually under the influence of the weight of the material in the tube. Depending on the material being packaged and the packaging process, the jaws of the sealing device are closed again. The tube collapses as a result at a second transverse section that is at, above, or below the air/material interface in the tube. The sealing device seals and severs the tube transversely at the second transverse section.
The material-filled portion of the tube is now in the form of a pillow shaped pouch. Thus, the sealing device has sealed the top of the filled pouch, sealed the bottom of the next-to-be-formed pouch and separated the filled pouch from the next-to-be-formed pouch, all in one operation.
One VFFS apparatus of the type described above is a Prepac® IS-7E liquid packaging apparatus. A commonly used sealing device, the so-called impulse sealer, has electrically insulated sealing element mounted in the sealing jaws. In operation, the sealing jaws close and simultaneously an electrical current flows through the sealing element, for example, a wire. The jaws remain closed while the seal forms, but not while it cools and solidifies. Therefore, once the sealing jaws open, the synthetic thermoplastic film must provide a molten transverse seal that supports the weight of the flowable material, for example, liquid, in the next-to-be-formed pouch.
For reasons of economy, customers are demanding thinner and thinner films for the pouching of fluids. This can lead to one of two problems in commercially available film formulations: (1) inadequate sealability and toughness, or (2) insufficient stiffness.
Pouches made from commercially available films tend to suffer from defective seals, that is, a tendency to have weak transverse end and/or longitudinal seals even though the operating conditions of the impulse sealer have been optimized. Defective seals may lead to the phenomenon known as “leakers,” in which the flowable material, for example, milk, may escape from the pouch through pinholes that develop at or close to the seal. It has been estimated that leakers account for about 1-2% of the 1.3 liter milk pouch production.
As the pouch film is down-gauged for reasons of economy, its stiffness may also become an issue. Lack of stiffness may adversely affect the runnability of the film on a form, fill and seal apparatus and give poor stand-up properties for pouches in, for instance, a milk pitcher. However, higher stiffness has traditionally required a thicker gauge of the pouch film. But thicker gauge requires more material. The present invention, by localizing the stiffening material towards the outside of the multi-layer film, addresses both these issues in that the stiffness of the multi-layer film is not adversely affected even when its gauge thickness is reduced.